Mate limitation in an estuarine population of copepods

We determined the probability of mating as a function of population density in the estuarine copepod Acartia hudsonica by combining experimental measurements with a simple model. Pairs of unmated copepods were confined in containers of various volumes to simulate variable population density, and experiments were run for 8, 16, and 24 h. Mating frequencies indicated that males search for females at an effective search volume rate of 0.34 6 0.15 L h21 or 8.2 6 3.5 L d21 and that males become ready to mate only after ,15 h exposure to the females. We applied these parameters in a simple population model to determine the critical density for zero population growth. With high egg production, zero mortality, and residence time of 60 d, the critical density for A. hudsonica was 0.01 m23, at the low end of the range of observed population densities. Critical densities for less favorable conditions were well within the range of observed population densities, even allowing for the effects of aggregation. Thus, mate limitation in sexually reproducing organisms, or Allee effects, can cause negative density dependence in growth rate of these populations at low but realistic population densities. We applied these results to the introduction of exotic zooplankton via ships’ ballast water under various scenarios of initial dilution in harbors and subsequent conditions for reproduction and survival. Inocula resulting from the discharge of postexchange ballast water were often high enough to establish new resident populations. Mate limitation at low population levels can result in negative or depensatory density dependence, a form of positive feedback by which reproductive success declines as the population shrinks (Allee 1931). When reproductive success is low, mortality and dispersive losses can lead to local extinction. Therefore, mate limitation is an important factor for species that are rare, that undergo seasonal minima in abundance, or that invade a new habitat. Until recently, mate limitation has been a neglected aspect in studies of population dynamics of marine organisms (Kiørboe 2006, 2007). We focus here on invasion success, although these concepts and methods apply as well to rare species. Successful invasions require a sufficient number of founding organisms that are physiologically robust enough to develop and reproduce (Williamson 1996; Kolar and Lodge 2001). Invasions by sexually reproducing organisms also require successful mating and reproduction following initial release. Few studies have examined the probable range of critical population density above which successful invasions are possible (e.g., Drake et al. 2005). Copepods are major invaders in estuarine ecosystems (Ferrari and Orsi 1984; Cordell and Morrison 1996). Concern over invasions by copepods centers on their ecological role in altering aquatic food webs and their possible role as vectors for potentially harmful microorganisms (e.g., Vibrio cholerae, Huq et al. 1983). Many marine bioinvasions have been mediated by ships’ ballast water, in which copepods are often the most abundant zooplankton and the genus Acartia is common (Levings et al. 2004; Choi et al. 2005). Copepods reproduce only sexually and have a well-described life history (Mauchline 1998). Thus, copepods are ideal model organisms for investigating invasion processes and mate limitation. The behavioral aspects of mating and particularly mate detection have been described for several copepod species. Generally, male copepods search for females, increasing detection range by tracking pheromone trails in some species or hydrodynamic disturbance in other species (Katona 1973; Bagøien and Kiørboe 2005; Kiørboe and Bagøien 2005). Most studies of mating behavior have focused on detection and sometimes pursuit, but few have addressed the other components of successful mating, namely, capture and copulation (Buskey 1998; Kiørboe 2007). Therefore the available data are inadequate to predict the population density at which Allee effects occur. We examined the influence of population density on mating success of Acartia hudsonica, an abundant omnivorous copepod. Members of this genus are usually biomass dominants in temperate estuaries. The ecology and behavior of Acartia species have been well studied (Mauchline 1998), and these species are easy to maintain in the laboratory. We combined a series of laboratory mating experiments with a model of the mating process to determine search volume rate (Kiørboe and Bagøien 2005) of the male copepods. These results were then used in a simple population model to determine how the critical density for population maintenance depends on reproductive rate and mortality.